US 7693039 B2 Abstract An apparatus and method for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system are provided for correcting an initial carrier frequency offset in the OFDM system. A metric generator for frequency estimation performs a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a frame, acquires a differential symbol from a product of adjacent FFT output symbols, performs a second accumulation process for a real part extracted from the differential symbol, and outputs a metric value for the frequency estimation. A maximal value-related index generator compares metric values for initial frequency estimation within a predetermined frequency offset estimation range, and selects and outputs a maximal metric value as a frequency offset estimate.
Claims(41) 1. An apparatus for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, the apparatus comprising:
a metric generator for frequency estimation for performing a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame, acquiring a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for the frequency estimation; and
a maximal value-related index generator for comparing metric values for initial frequency estimation within a frequency offset estimation range and selecting and outputting a maximal metric value as a frequency offset estimate.
2. The apparatus of
a frequency offset corrector for correcting a frequency offset of data received by the reception stage in accordance with the frequency offset estimate outputted from the maximal value-related index generator.
3. The apparatus of
a Pseudo Noise (PN) detector for multiplying the PRS generated from the reception stage by the FFT output signal for the OFDM symbol in the PRS position within the predefined frame;
a first accumulator for performing the first accumulation process for an output of the PN detector;
a differential symbol detector for outputting the differential symbol using the product of the adjacent FFT output symbols;
a real part detector for extracting the real part from the differential symbol; and
a second accumulator for performing the second accumulation process for an output of the real part detector during a predetermined interval.
4. The apparatus of
a Pseudo Noise (PN) detector for multiplying the PRS generated from the reception stage by the FFT output signal for the OFDM symbol in the PRS position within the predefined frame;
a first accumulator for performing the first accumulation process for an output of the PN detector;
a differential symbol detector for outputting the differential symbol using the product of the adjacent FFT output symbols;
a magnitude generator for extracting a magnitude component from the differential symbol; and
a second accumulator for performing the second accumulation process for an output of the magnitude generator.
5. The apparatus of
where Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.6. The apparatus of
where p is an integer more than 0, Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.7. The apparatus of
where f
_{n }is an integer multiple of a carrier frequency offset estimate and Z(f_{n}) is the metric value for the frequency estimation.8. The apparatus of
9. An apparatus for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, the apparatus comprising:
a metric generator for frequency estimation for performing a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame, acquiring a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for the frequency estimation; and
a threshold comparator for determining whether the metric value for initial frequency estimation exceeds a threshold, and outputting the metric value as a frequency offset estimate when the metric value exceeds the threshold.
10. The apparatus of
a frequency offset corrector for correcting a frequency offset of data received by the reception stage in accordance with the frequency offset estimate outputted from the threshold comparator.
11. The apparatus of
a Pseudo Noise (PN) detector for multiplying the PRS generated from the reception stage by the FFT output signal for the OFDM symbol in the PRS position within the predefined frame;
a first accumulator for performing the first accumulation process for an output of the PN detector;
a differential symbol detector for outputting the differential symbol using the product of the adjacent FFT output symbols;
a real part detector for extracting the real part from the differential symbol; and
a second accumulator for performing the second accumulation process for an output of the real part detector during a predetermined interval.
12. The apparatus of
a first accumulator for performing the first accumulation process for an output of the PN detector;
a magnitude generator for extracting a magnitude component from the differential symbol; and
a second accumulator for performing the second accumulation process for an output of the magnitude generator.
13. The apparatus of
where Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.14. The apparatus of
where p is an integer more than 0, Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.15. The apparatus of
16. A method for carrier frequency synchronization by a carrier frequency synchronization apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system, the method comprising:
performing, by the carrier frequency synchronization apparatus, a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame;
acquiring, by the carrier frequency synchronization apparatus, a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for frequency estimation; and
comparing, by the carrier frequency synchronization apparatus, metric values for initial frequency estimation within a frequency offset estimation range and selecting and outputting a maximal metric value as a frequency offset estimate.
17. The method of
correcting, by the carrier frequency synchronization apparatus, a frequency offset of data received by the reception stage in accordance with the selected frequency offset estimate.
18. The method of
where Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.19. The method of
where p is an integer more than 0, Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.20. The method of
where f
_{n }is an integer multiple of a carrier frequency offset estimate and Z(f_{n}) is the metric value for the frequency estimation.21. The method of
22. A method for carrier frequency synchronization by a carrier frequency synchronization apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system, the method comprising:
performing, by the carrier frequency synchronization apparatus, a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame;
acquiring, by the carrier frequency synchronization apparatus, a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for frequency estimation; and
determining, by the carrier frequency synchronization apparatus, whether the metric value for initial frequency estimation exceeds a threshold, and selecting and outputting the metric value exceeding the threshold as a frequency offset estimate.
23. The method of
correcting, by the carrier frequency synchronization apparatus, a frequency offset of data received by the reception stage in accordance with the selected frequency offset estimate.
24. The method of
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.25. The method of
where p is an integer more than 0, Y[k] is a k-th FFT output result for the OFDM symbol in the PRS position, f
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.26. The method of
27. A method for carrier frequency synchronization by a carrier frequency synchronization apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system, the method comprising:
performing, by the carrier frequency synchronization apparatus, a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame;
acquiring, by the carrier frequency synchronization apparatus, a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for frequency estimation;
determining, by the carrier frequency synchronization apparatus, whether a frequency index related to the metric value for the frequency estimation is last; and
selecting and outputting, by the carrier frequency synchronization apparatus, a frequency index comprising a maximal value of metric values stored in frequency indexes as a frequency offset estimate when the related frequency index is determined to be last.
28. The method of
correcting, by the carrier frequency synchronization apparatus, a frequency offset of data received by the reception stage in accordance with the frequency offset estimate.
29. The method of
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.30. The method of
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.31. The method of
where f
_{n }is an integer multiple of a carrier frequency offset estimate and Z(f_{n}) is the metric value for the frequency estimation.32. The method of
33. A method for carrier frequency synchronization by a carrier frequency synchronization apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system, the method comprising:
performing, by the carrier frequency synchronization apparatus, a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame; and
acquiring, by the carrier frequency synchronization apparatus, a differential symbol from a product of adjacent FFT output symbols, performing a second accumulation process for a real part extracted from the differential symbol, and outputting a metric value for frequency estimation.
34. The method of
35. The method of
36. The method of
determining, by the carrier frequency synchronization apparatus, whether a frequency index related to the metric value for the frequency estimation is last; and
selecting and outputting, by the carrier frequency synchronization apparatus, a frequency index comprising a maximal value of metric values stored in frequency indexes as a frequency offset estimate when the related frequency index is determined to be last.
37. The method of
correcting, by the carrier frequency synchronization apparatus, a frequency offset of data received by the reception stage in accordance with the frequency offset estimate.
38. The method of
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k−f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.39. The method of
_{n }is an integer multiple of a carrier frequency offset estimate, p[k−f_{n}] is a local PRS of a receiver with respect to f_{n}, p*[k−f_{n}] is a complex conjugate of p[k −f_{n}], R*[m+1] is a complex conjugate of R[m+1], and N_{1 }is an accumulation length of the first accumulation process.40. The method of
_{n }is an integer multiple of a carrier frequency offset estimate and Z(f_{n}) is the metric value for the frequency estimation.41. The method of
Description This application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Nov. 29, 2005 and assigned Serial No. 2005-115153, the entire disclosure of which is hereby incorporated by reference. 1. Field of the Invention The present invention generally relates to a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). More particularly, the present invention relates to an apparatus and method for correcting an initial carrier frequency offset in a wireless communication system based on OFDM. 2. Description of the Related Art Wireless communication systems typically make use of a cellular communication scheme. These wireless communication systems make use of multiple access schemes for simultaneous communication with multiple users. For the multiple access schemes, Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) are typically used. With the rapid progress of CDMA technology, CDMA systems are developing from a voice communication system into a system capable of transmitting packet data at high speeds. In order to overcome limitations in using code resources of the CDMA system, an Orthogonal Frequency Division Multiple Access (OFDMA) scheme has been used recently. The OFDMA scheme is based on Orthogonal Frequency Division Multiplexing (OFDM). An OFDM system for transmitting data using multi-carrier is a type of Multi Carrier Modulation (MCM) system in which a serial symbol stream is converted into parallel symbol streams and is modulated into multiple subcarriers, that is, multiple subcarrier channels, orthogonal to each other. The MCM-based OFDM scheme was first applied to High Frequency (HF) radio communications for the military in the late 1950's. The OFDM scheme for overlapping orthogonal subcarriers started to be developed in the 1970's. Since a problem exists in that it is difficult to implement orthogonal modulation between multiple carriers, the OFDM scheme has limitations in actual system implementation. However, in 1971, Weinstein, et al. proposed that OFDM modulation/demodulation can be efficiently performed using Discrete Fourier Transform (DFT). Thus, the OFDM technology has rapidly developed. Also, the introduction of a guard interval into which a Cyclic Prefix (CP) symbol is inserted further mitigates adverse effects of multipath propagation and delay spread on an OFDM system. As a result, with the development of technology, the OFDM scheme has been widely used for digital transmission technologies such as Digital Audio Broadcasting (DAB), digital television (TV), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode, (WATM), and the like. Although hardware complexity is an obstacle to implementation of the OFDM system, recent advances in digital signal processing technology including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) enable the OFDM system to be implemented. The OFDM scheme is analogous to a conventional Frequency Division Multiplexing (FDM) scheme, and can obtain optimal transmission efficiency when high-speed data is transmitted while maintaining orthogonality between multiple subcarriers. More specifically, the OFDM scheme leads to efficient frequency use and is robust to multipath fading, thereby obtaining optimum transmission efficiency upon transmission of high-speed data. The OFDM scheme uses overlapping frequency spectra, thereby efficiently using frequencies. The OFDM scheme is robust to frequency selective fading, multipath fading, and impulse nose. The OFDM scheme can reduce Inter Symbol Interference (ISI) using guard intervals and can easily design an equalizer structure in hardware. Therefore, the OFDM scheme is actively exploited in communication system structures. An input bit stream An OFDM symbol is input to a CP adder The transmitted analog symbols are input to the receiver through a predefined multipath channel An Analog-to-Digital Converter (ADC) The serially input symbols are converted into the parallel symbols in the unit of N symbols because the transmitter performs the IFFT process in the unit of N symbols. Thus, an FFT processor The above-described OFDM system can more efficiently use a transmission band in comparison with a single carrier modulation system. For this reason, the OFDM system is widely used for a broadband transmission system. In terms of reception characteristics, the OFDM system is more robust to a frequency selective multipath fading channel in comparison with a single carrier transmission system. Because there are a frequency selective channel in a frequency band occupied by multiple subcarriers and a frequency nonselective channel in each subcarrier band in terms of input signal characteristics of a receiver, a channel can be easily compensated in a simple channel equalization process. In particular, the OFDM system copies a second half part of each OFDM symbol, attaches the copied part as a CP before the OFDM symbol, and transmits the OFDM symbol, thereby removing ISI from a previous symbol. Thus, the OFDM transmission scheme is robust to the multipath fading channel and is proper for broadband high-speed communication. In a standard for digital broadcasting, the OFDM transmission scheme receives attention as a transmission scheme capable of ensuring high quality of reception and high-speed transmission and reception. Examples of broadcasting standards adopting the OFDM transmission scheme are DAB for European wireless radio broadcasting, Digital Video Broadcasting-Terrestrial (DVB-T) serving as a terrestrial High Definition Television (HDTV) standard, and the like. Recently, a mobile broadcasting system is being developed in line with the global trend towards the convergence of broadcasting and communications. In particular, a major object of the mobile broadcasting system is to transmit a large amount of multimedia information. In Europe, DVB-Handheld (DVB-H) developed from DVB-T has been adopted as the mobile broadcasting standard. In South Korea, terrestrial Digital Multimedia Broadcasting (DMB) developed from DAB has been adopted as the broadcasting standard along with European DVB-H. MediaFLO proposed by Qualcomm is also based on the OFDM transmission scheme. When a reception stage receives a signal modulated and transmitted by a transmission stage and converts the received signal into a baseband signal, synchronization between a transmission frequency and a reception frequency may not be acquired due to a tuner characteristic difference between the transmission stage and the reception stage. Herein, a frequency difference is referred to as a frequency offset. Because this frequency offset leads to a decrease in signal magnitude and interference between adjacent channels, its correction is important to determine the performance of the OFDM system. To correct the frequency offset in the OFDM scheme, many algorithms have been proposed. Synchronization algorithms for the OFDM system are divided into a carrier frequency synchronization algorithm and a symbol timing synchronization algorithm. The carrier frequency synchronization algorithm performs a function for correcting a carrier frequency offset between a transmitter and a receiver. The carrier frequency offset is caused by an oscillator frequency difference between the transmitter and the receiver, and a Doppler frequency offset. The carrier frequency offset of a signal input to a reception stage may be more than a subcarrier interval. A process for correcting an associated carrier frequency offset corresponding to an integer multiple of the subcarrier interval is defined as “initial carrier frequency synchronization.” A process for correcting an associated carrier frequency offset corresponding to a decimal multiple of the subcarrier interval is defined as “fine carrier frequency synchronization. A transmitted OFDM signal is shifted by an integer multiple of a subcarrier unit in a frequency domain due to an offset corresponding to an integer multiple of a subcarrier unit and therefore an FFT output sequence is shifted by the integer multiple of the subcarrier unit. On the other hand, the carrier frequency offset corresponding to the decimal multiple of the subcarrier leads to interference between FFT outputs and significant degradation of Bit Error Rate (BER) performance. In general, it is known that the OFDM system has a larger amount of performance degradation due to the carrier frequency offset in comparison with the single carrier transmission system. Existing initial carrier frequency synchronization algorithms for the OFDM system can be divided into a blind detection algorithm and an algorithm using a predefined symbol. In an example of the blind detection algorithm, a shift amount of a signal band is estimated using a guard band. However, it is difficult to actually implement the blind detection algorithm because performance degradation is very large under a multipath fading channel environment. On the other hand, the algorithm using the predefined symbol is disadvantageous in that a data transmission rate is reduced because the predefined symbol is transmitted independent of a data symbol. However, the algorithm using the predefined symbol is widely used for many OFDM systems because the performances of synchronization and channel estimation are improved. In general, the predefined symbol transmitted for synchronization and channel estimation of the reception stage is constructed with a sequence capable of using autocorrelation characteristics like a Pseudo Noise (PN) sequence. As the initial carrier frequency synchronization algorithm using the predefined symbol, algorithms proposed by Nogami and Taura are well known. The algorithm proposed by Nogami is illustrated in First, a PN detector The metric value Z
Herein, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, f Because the algorithm proposed by Nogami as illustrated in Referring to The metric value Z
Herein, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, f On the other hand, the algorithm proposed by Taura corrects a PN sequence in a frequency domain, transforms the frequency domain sequence into a time domain sequence, and estimates a frequency shift amount mapped to a maximal value as an initial carrier frequency offset. This algorithm is significantly robust to a symbol timing offset, but requires very high hardware complexity because an IFFT process should be performed to compute every frequency offset estimate. Among the conventional initial carrier frequency synchronization technologies in an OFDM receiver, the algorithm proposed by Nogami is difficult to be applied because autocorrelation characteristics are degraded when an FFT timing offset is large in a reception stage. That is, the FFT timing offset leads to linear phase rotation in the frequency domain. Thus, an autocorrelation length is reduced due to a limitation in the number of subcarriers capable of taking autocorrelation. As the autocorrelation length decreases, an autocorrelation value decreases and detection performance is degraded even though noncoherent combining is performed because distortion easily occurs due to a noise component. If an offset value is very large although FFT timing is detected, it can be seen that the performance of initial carrier frequency synchronization acquisition is significantly degraded in Nogami's algorithm. On the other hand, when the FFT timing offset of the reception stage is small and interference from a previous symbol is absent under a multipath channel environment, only multipath components with a relatively small timing offset provide a large autocorrelation value and only multipath components with a relatively large timing offset provide a small autocorrelation value. In a Single Frequency Network (SFN) and a multipath channel environment with large channel delay spread, an amount of performance degradation further increases in Nogami's algorithm. Among the conventional initial carrier frequency synchronization technologies in the OFDM receiver, the algorithm proposed by Taura can detect a predefined symbol even when an FFT timing offset is large, but has a disadvantage in that an IFFT process with very high hardware complexity should be used for processing in the time domain. In particular, the algorithm proposed by Taura is difficult to be used when a frequency offset is large because the IFFT process should be performed for one frequency estimate. Because only a multipath component with a largest magnitude value is used after transformation into the time domain, the number of multiple paths increases. There is a disadvantage in that performance is significantly degraded when the magnitudes of multipath components are similar to each other. Accordingly, there is a need for an improved apparatus and method for carrier frequency synchronization in an OFDM system that sustains performance in the presence of multipath interference. An aspect of exemplary embodiments of the present invention is to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of exemplary embodiments of the present invention is to provide an apparatus and method for carrier frequency synchronization that can improve the performance of initial carrier frequency offset detection and the degradation of autocorrelation characteristics in an environment where an Fast Fourier Transform (FFT) timing offset of a reception stage is large in a wireless communication system based on Orthogonal Frequency Division Multiplexing (OFDM). It is another aspect of exemplary embodiments of the present invention to provide an apparatus and method for carrier frequency synchronization that can obtain autocorrelation characteristics by sufficiently employing all multipath components even under a Single Frequency Network (SFN) environment and a multipath channel environment where channel delay spread is large in a wireless communication system based on OFDM. It is another aspect of exemplary embodiments of the present invention to provide an apparatus and method for carrier frequency synchronization in a wireless communication system based on OFDM that can further reduce hardware complexity by processing a signal in a frequency domain in comparison with a conventional system having high hardware complexity. It is yet another aspect of exemplary embodiments of the present invention to provide an apparatus and method for carrier frequency synchronization that can be robust to a symbol timing offset and a multipath channel environment while employing a simple hardware structure in a wireless communication system based on OFDM. In accordance with an aspect of exemplary embodiments of the present invention, there is provided an apparatus for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, in which, a metric generator for frequency estimation performs a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame, acquires a differential symbol from a product of adjacent FFT output symbols, performs a second accumulation process for a real part extracted from the differential symbol, and outputs a metric value for the frequency estimation; and a maximal value-related index generator compares metric values for initial frequency estimation within a predetermined frequency offset estimation range, and selects and outputs a maximal metric value as a frequency offset estimate. In accordance with another aspect of exemplary embodiments of the present invention, there is provided an apparatus for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, in which, a metric generator for frequency estimation performs a first accumulation process for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame, acquires a differential symbol from a product of adjacent FFT output symbols, performs a second accumulation process for a real part extracted from the differential symbol, and outputs a metric value for the frequency estimation; and a threshold comparator determines whether metric values for initial frequency estimation exceed a specific threshold, and selects and outputs a metric value exceeding the specific threshold as a frequency offset estimate. In accordance with another aspect of exemplary embodiments of the present invention, there is provided a method for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, in which, a first accumulation process is performed for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame; a differential symbol is acquired from a product of adjacent FFT output symbols, a second accumulation process for a real part extracted from the differential symbol is performed, and a metric value for frequency estimation is output; and metric values for initial frequency estimation within a predetermined frequency offset estimation range and selecting and outputting a maximal metric value as a frequency offset estimate are compared. In accordance with another aspect of exemplary embodiments of the present invention, there is provided a method for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, in which, a first accumulation process is performed for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame; a differential symbol from a product of adjacent FFT output symbols is acquired, a second accumulation process for a real part extracted from the differential symbol is performed, and a metric value for frequency estimation is output; and whether metric values for initial frequency estimation exceed a specific threshold are determined, and a metric value exceeding the specific threshold as a frequency offset estimate is selected and output. In accordance with yet another aspect of exemplary embodiments of the present invention, there is provided a method for carrier frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system, in which, a first accumulation process is performed for a value computed by multiplying a Phase Reference Symbol (PRS) generated from a reception stage by a Fast Fourier Transform (FFT) output signal for an OFDM symbol in a PRS position within a predefined frame, a differential symbol from a product of adjacent FFT output symbols is acquired, a second accumulation process for a real part extracted from the differential symbol is performed, and a metric value for frequency estimation is output; whether a frequency index related to the metric value for the frequency estimation is last is determined; and a frequency index, having a maximal value of metric values stored in frequency indices, is selected and output as a frequency offset estimate when the related frequency index is determined to be last. The above and other objects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features and structures. The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of exemplary embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. In an Orthogonal Frequency Division Multiplexing (OFDM) system, operations of an initial carrier frequency estimator can be divided into a case where correction is made in a digital domain as illustrated in Referring to The ADC The frequency offset corrector A Fast Fourier Transform (FFT) processor The frequency offset estimator On the other hand, in For symbol and carrier synchronization processes, a NULL symbol In a synchronization process of the DAB system, frame synchronization is performed by detecting the NULL symbol The OFDM system is provided with a memory (not illustrated), a Pseudo Noise (PN) detector In the initial carrier frequency estimator of the OFDM system in accordance with an exemplary embodiment of the present invention, the memory stores a result obtained by receiving an OFDM symbol in a position of the predefined symbol and performing an FFT process for the received OFDM symbol. The PN detector
Herein, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, f
Herein, p is an integer more than 0, Y[k] is a k-th FFT output result for an OFDM symbol in a PRS position, f Equations (3) and (4) indicate the metric value for the integer multiple of the carrier frequency offset estimate f
On the other hand, the maximal value-related index detector A carrier frequency synchronization method in a wireless communication system based on OFDM in accordance with an exemplary embodiment of the present invention will be described with reference to In step In step In step A carrier frequency synchronization method in a wireless communication system based on OFDM in accordance with an exemplary embodiment of the present invention will be described with reference to In step In step However, if Z In step Exemplary embodiments of the present invention can significantly improve the performance of initial carrier frequency offset detection by further improving the degradation of autocorrelation characteristics even in an environment where an FFT timing offset of a reception stage is large in comparison with the conventional method. Exemplary embodiments of the present invention can employ a differential symbol detection structure and obtain autocorrelation characteristics by more sufficiently employing all multipath components even in a multipath channel environment where channel delay spread is large, thereby improving the performance of initial carrier frequency offset detection. Exemplary embodiments of the present invention can further reduce hardware complexity by processing a signal in a frequency domain in comparison with a conventional system having high hardware complexity. Conventionally, frame or timing synchronization is performed such that an FFT timing offset is sufficiently small. However, exemplary embodiments of the present invention can roughly perform frame and/or timing synchronization. While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents. Patent Citations
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